Advances in Optical and Photonic Devices 2011 Part 11 pptx

Quantum Dot Photonic Devices and Their Material Fabrications 239 1042.71 nm–ch.4: 1043.85 nm). Each of the central wavelengths is selected for the 100-GHz channel spacing of the AWG by using the discrete single-mode selection method of the QD- CML. Figure 9(b) shows a typical eye diagram at ch. 2 after transmission. A clear eye opening at 12.5 Gbps is observed after the transmission. Therefore, the 1-μm waveband with a 12.5-Gbps transmission over a long-distance (1.5 km) single-mode HF is successfully YDFA LN Modulator 1-µm-waveband single-mode holey-fiber Distance: 1.48 km 1-µm waveband Quantum dot comb-laser (QD-CML) with mode-selection YDFA PPG 1-µm waveband arrayed waveguide grating YDFA 0.6-nm OSA 0.6-nm Communications analyzer 12.5-Gbps 0 dBm Fig. 8. Experimental set-up for testing the 1-μm WDM photonic transport system. A 1-μm waveband and single-mode selected quantum dot optical-frequency comb laser (QD-CML) was used for the light source. 1041 1042 1043 1044 1045 -70 -60 -50 -40 -30 -20 -10 0 10 * Arrayed-Waveguide Grating (AWG) for 1-micron waveband, 100 GHz spacing * Injection seeded Sb-based QD FP-LD Ch4Ch3Ch2Ch1 Recieved power (dBm) Wavelength (nm) After 1.5 km transmission 20 ps/div 1043.2 nm (Ch2) After 1.5 km transmission 20 ps/div 1043.2 nm (Ch2) (b) (a) Fig. 9. (a) Optical spectrum of 12.5-Gbps and single-mode selected QD-CML after 1.5-km transmission of the holey fiber. (b) Eye opening of ch.2 after transmission. Advances in Optical and Photonic Devices 240 achieved at four different wavelengths by using a wavelength-tunable discrete single-mode selected QD laser device. The 1-μm waveband AWG, YDFAs, and other passive devices are also important to construct the 1-μm waveband photonic transport system. From these results, a 12.5-Gbps-based WDM photonic transmission with a 100-GHz channel spacing can be realized in the 1-μm waveband by using the proposed methods. Additionally, it is expected that the QD photonic devices such as a semiconductor laser fabricated on the GaAs wafer will become a powerful candidate to realize an ultra-broadband 1- to 1.3-μm photonic transport system. 3. Quantum dot structure for advanced photonic devices In this section, novel material systems of a QD structure are introduced for advanced photonic devices. The novel materials of the QD are expected to be used in laser device fabrication, silicon photonics, visible light-emitting devices, etc. 3.1 Long-wavelength quantum dot structure Sb-based III-V semiconductor materials have very narrow-band gap properties. Therefore, the use of Sb-based III-V semiconductor QD structures (the Sb atoms are included in the QD structure) are expected for producing long-wavelength-emitting devices (Yamamoto et al. 2005 & 2006b). In this section, the Sb-based QD structure fabricated on a GaAs substrate is introduced. However, the fabrication of the Sb-based QD such as an InGaSb QD is difficult under conventional QD growth conditions with the MBE method. To form the high-quality Sb-based QD structure, a Si atom irradiation technique is proposed as one of the methods for surface treatment. Figure 10(a) shows a schematic image of the Si atom irradiation GaAs GaAs GaAs GaAs Reducing surface free energy :σ s Enhanced S-K growth mode:σ s <σ f (σ f : Film free-energy) High-density Sb-based QD structure Silicon atom irradiation technique Silicon In, Ga and Sb InGaSb QD (a) (b) (c) Fig. 10. (a) Schematic image of silicon atom irradiation technique for the fabrication of the high-quality QD structure. AFM images of InGaSb QD structure in a 5 × 5 -μm 2 region on GaAs substrate without (b) and with (c) the Si atom irradiation technique. Quantum Dot Photonic Devices and Their Material Fabrications 241 technique. Low density Si atoms are irradiated on to the GaAs surface immediately before the Sb-based QD structure growth. It is expected that the surface free-energy may be reduced with the irradiation of Si atoms. Therefore, the density of the Sb-based QD structure is enhanced by using this atom-irradiation technique. Figures 10(b) and (c) show the AFM images of the Sb-based QD structure without and with the Si atom irradiation, respectively. It is found that the QD density with Si atoms is approximately 100 times higher than that without Si atoms. Generally, the QD density as high as 10 10 /cm 2 is necessary if the QD structure is used for developing a laser or other photonic devices. Therefore, the optimization of the QD growth conditions such as growth-rate, As-flux intensity, and temperature is also important to obtain the high-quality QD structure. Figure 11(a) shows an AFM image of the Sb-based QD/GaAs structure under the optimized growth conditions. The height, dimension, and density of the Sb-based QD are approximately 7.5 nm, 25 nm, and 2 × 10 10 /cm 2 , respectively. An ultra-wideband emission between wavelengths of 1.08- and 1.48-μm can be successfully realized by using the Sb-based QD/GaAs structure, as shown in Fig. 11(b). The long- wavelength and ultra-broadband emission is also obtained from a light-emitting diode (LED) that contained the Sb-based QD in active regions. From this result, it is expected that ultra-broadband wavelength (>350 nm) light sources may be achieved with the QD structure for the O-, E-, S-, and C-band (Yamamoto et al. 2009a). 1000 1200 1400 1600 Emission (dB) Ultra-wideband InGaSb QDs with Si atom irradiation technique at Room temperature Wavelength (nm) (b)(a) Fig. 11. (a) Atomic force microscope image of high-quality Sb-based QD (InGaSb QD) structure on GaAs surface. (b) Ultra broadband and long-wavelength emission from the Sb- based QD/GaAs structure. The combination of a micro-cavity structure and the QD structure is a very interesting device structure for the investigation of cavity quantum-electrodynamics (QED). Study on the QED of the QD structure is important for constructing a quantum communications system (Ishi-Hayase et al. 2007 & Kujiraoka et al. 2009). A vertical cavity structure and a photonic crystal structure as an optical resonator are useful for confining the photons (Nomura et al. 2009). Figure 12(a) presents a cross-sectional image of a fabricated vertical Advances in Optical and Photonic Devices 242 cavity structure, which include the Sb-based QD in the cavity. A high-performance diffractive Bragg reflector (DBR) for accomplishing the vertical cavity structure can be simply produced by using an AlGaAs material system. From the Sb-based QD structure in the vertical cavity, a 1.55-μm sharp emission peak, as shown in Fig. 12(b), is successfully observed under the optically pumped condition (Yamamoto et al. 2006a). It is also found that a long-wavelength emission with a 1.52-μm peak can be obtained from the similar QD in the cavity structure at room temperature with a current injection. Therefore, it is expected that the use of the long wavelength QD active media in the semiconductor micro-cavity structure is a very useful and important way for fabricating long-wavelength and multiwavelength vertical cavity surface emitting lasers (VCSELs), resonant cavity light- emitting diodes (RCLEDs), single photon sources, etc. n- doped　 GaAs/AlGaAs DBR mirrors p- doped　 GaAs/AlGaAs DBR mirrors Cross-sectional image of vertical cavity structure Stacked InGaSb QDs active layer Sb-based Quantum Dot (a) (b) Fig. 12. (a) Sb-based QD in micro cavity structure and (b) 1.55-μm wavelength emission spectrum from optically pumped vertical cavity structure. 3.2 Quantum dot and related materials for silicon photonics Silicon photonics technology has been conventionally used to fabricate high performance photonic circuits, which have low-power-consumption, are compact, and are relatively inexpensive to fabricate (Liu et al. 2004 & Yamamoto et al. 2007b). Poly-, amorphous-, and crystalline-Si waveguide devices have been developed and their properties have been investigated. An optical gain region must be provided for silicon waveguide structures to enable the fabrication of active devices such as light emitters and optical amplifiers on silicon platforms (Balakrishnan et al. 2006). As one of the candidates of the optical gain media, a III-V semiconductor QD structure on a Si wafer has been investigated. Figure 13 shows the schematic image of the Sb-based QD/Si structure and AFM images of the Sb- based QD structures grown between 400°C and 450°C on Si substrates (Yamamoto et al. 2007a). From the AFM image, it is found that the high-quality and high-density Sb-based QD structure can be obtained under the optimal growth conditions by MBE. Therefore, a Quantum Dot Photonic Devices and Their Material Fabrications 243 high-density (>10 10 /cm 2 ) and small-sized (<10 nm) QD structure can be obtained by growing the QDs below 400°C. From this result, it is expected that the nanostructured Sb- based semiconductors with a low-temperature process (<400°C) should become useful materials for complementary metal oxide semiconductor (CMOS) devices compatible with silicon photonics technology (Yamamoto et al. 2008a). Additionally, it is also expected that the nanostructured Sb-based semiconductor will be used for high-speed electro-devices, because the III-Sb compound semiconductor has high-mobility characteristics (Ashley et al., 2007). Silicon (001) InGaSb QD (b) (c) (a) Fig. 13. (a) Schematic image of Sb-based QD structure on Si wafer, and AFM images of the Sb-based QD on Si at (b) 400°C and (c) 450°C. Compound semiconductors are widely studied for the fabrication of the QD structure because they exhibit an observable quantum size effect in the quantum confinement structure of a relatively large size (approximately few tens of nanometers). On the other hand, a carrier confined structure several nanometers in size, which is generally called a nanoparticle, is necessary when using a silicon semiconductor material. Several techniques have been proposed for the fabrication of the Si nanoparticle as a Si-QD structure (Canham et al. 1990). An anodization method and a photochemical etching method of a Si wafer are proposed for producing the Si nanoparticles (Yamamoto et al. 2001 & Hadjersi et al. 2004). It is known that the Si nanoparticle exhibits a bright visible light emission of red or blue color, and it is considered that this light emission is caused by the quantum size effect of the Si- QD. Figure 14(a) shows a visible emission spectrum from the photochemically etched layers, such as Si nanoparticles (Yamamoto et al. 1999). In addition, electroluminescence devices on a Si wafer are also demonstrated using Si nanoparticles, as shown in Figure 14(b). It is expected that the Si nanoparticle as the Si-QD structure will become a useful material for the visible light-emitting devices with Si-based electric devices (Yamamoto et al. 2000). 4. Conclusion The quantum dot (QD) structures are intensively investigated as the three-dimensional carrier confined structure. It is expected that the QD structure can act likely as an atom, which has a controllable characteristic of energy levels. The semiconductor QD structure is a very important material for developing novel photonic devices. In this chapter, fabrication techniques and characteristics of novel QD photonic devices such as a broadband QD light Advances in Optical and Photonic Devices 244 500600700800 Area-B Area-A Area-BArea-A Si wafer Selective area formation of Photo-chemically etched silicon Normalized PL intensity Wavelength(nm) Visible electroluminescence Light emitting device by using photo-chemically etched Si Fig. 14. (a) Emission spectra of photochemically etched layers as Si nanoparticles. The emission colors in areas A and B are observed as yellow and red, respectively. Each layer is formed on the same Si substrate using a selective area formation technique. (b) Visible electroluminescence devices on Si wafer by using the Si-particle as the Si-QD. source and a wavelength tunable QD laser were explained. The QD light source act in a broad wavelength band between 1-μm and 1.3-μm can be fabricated on the GaAs substrate as a low cost and large-sized wafer by using InAs QD and InGaAs QD structures as an active media. In addition, a fabrication technique of the Sb-based QD structures on the GaAs substrate was demonstrated for the ultra-broadband light source between 1 and 1.55 μm, and the novel photonic devices using the cavity-QED. In other words, by using the QD structure, ultra-broadband optical gain media can be achieved for broadband light-emitting diodes, wavelength tunable laser diodes, semiconductor optical amplifiers, etc. Additionally, the QD structures have interesting opto-electric characteristics compared to the conventional quantum well and bulk materials. It is expected that the QD optical frequency comb laser (QD-CML) can be realized by using the useful characteristics of the QD structure. Ultra-broadband optical frequency resources in the short wavelength band such as the 1-μm waveband can be used for optical communications. As the 1-μm waveband photonic transport system, over 10 Gbps and a long distance transmission were successfully demonstrated by using high-performance key components such as single-mode QD light sources, long-distance holey fibers, and YDFAs. Therefore, it is expected that the uses of the QD photonic devices enhance the usable waveband for optical communications. For the silicon photonics, a fabrication technique for the high-quality Sb-based QD structure on a Si wafer was demonstrated clearly. As the other QD structure for the silicon photonics, it is also demonstrated that Si nanoparticles as the Si-QD become candidates for the light- emitting devices on the Si wafer. It is expected that a fabrication and application of the QD structure will provide a breakthrough technology for the creation of novel photonic devices, improvement in the Quantum Dot Photonic Devices and Their Material Fabrications 245 existing photonic devices, and enhancement of usable optical frequency resources in the all- photonic waveband. 5. Acknowledgments The authors would like to thank Prof. H. Yokoyama at New Industry Creation Hatchery Center (NICHe) of Tohoku University, Prof. H. Takai at Tokyo Denki University (TDU), Drs. K. Akahane, R. Katouf, T. Kawanishi, I. Hosako, and Y. Matsushima at the National Institute of Information and Communications Technology (NICT) for discussing novel technologies of the quantum dot photonic devices and lasers. The authors are deeply grateful to Drs. K. Mukasa, K. Imamura, R. Miyabe, T. Yagi, and S. Ozawa at FURUKAWA ELECTRIC CO. for discussing broadband transmission lines of the novel optical fibers. 6. References Akahane, K.; Yamamoto, N., Sotobayashi, H. & Tsuchiya, M. (2008). 1.7-μm Laser Emission at Room Temperature using Highly-Stacked InAs Quantum dots. Proceedings of Indium Phosphide and Related Material (IPRM) 2008, Versailles, 171 Arakawa, Y. & Sakaki, H. (1982). Multidimensional quantum well laser and temperature dependence of its threshold current. Appl. Phys. Lett. Vol. 40, 939 (1982) 939 Ashley, T.; Buckle, L.; Datta, S.; Emeny, M. T.; Hayes, D. G.; Hilton, K. P.; Jefferies, R.; Martin, T.; Phillips, T. J.; Wallis, D. J.; Wilding, P. J. & Chau, R. (2007). 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[...]... – self sustaining avalanche breakdown process 254 Advances in Optical and Photonic Devices in conventional avalanche photo detectors For high level of electric field in the silicon structure, process shown on Fig 3.b, impact ionization coefficients coming close to each other and both type of carriers electrons and holes could participate in the avalanche process and create self-sustaining avalanche... material and technologies compatible to the main mass production 258 Advances in Optical and Photonic Devices technology processes as CMOS technology and more important aspect that materials and technology allowed produce the integral device including the sensors and readout electronics on the same substrate In future the integrated silicon photomultiplier with readout electronics on the chip will dominated... across the silicon band gap As this wavelength is approached the probability of photon absorption decreases rapidly with increasing wavelength It will be noted that the absorption coefficient increases with increasing temperature leading to an increase in long wavelength responsivity with temperature Cut off at short wavelength occurs in silicon 262 Advances in Optical and Photonic Devices photomultiplier... photomultiplier tubes in term of detecting of low photon flux, but has a great advantages in performance and operation conditions and has great future in many areas of applications such as experimental physics, nuclear medicine, homeland security, military applications and other Silicon Photomultipliers shows the excellent performance including the single photon response at room temperature (intrinsic gain of multiplication... undergo avalanche breakdown In this state a single carrier entering the depletion region is enough to initiate avalanche multiplication process and produce a self-sustaining current The initiation could be as result of incoming photon interaction or termal created carrier inside depleted area For the stopping of the avalanche breakdown process, the quenching elements are implemented in the silicon photomultiplier... detecting the low photon flux or single photon by the semiconductor strictures, like silicon photomultipliers Nevertheless to rich the value of intrinsic gain of level 106 or more in semiconductor structures is not trivial task in development of silicon photomultipliers For remaining, the principle of internal gain of multiplication was realized in the Photomultiplier Tubes – electro vacuum devices, ... micro-cells detecting the photons across sensitive area and define the single photon detection resolution Finally intrinsically the silicon photomultiplier is completely digital device, which produce the number of equivalent charge pulse caused by photon interaction in the space distributed structure of equivalent micro-cells and integrated on the output and correspondent to the number of incoming photons... and interaction in the silicon structure – three photons interact in the three micro-cells and initiated the avalanche breakdown processes On Fig 4, b is presented correspondent electronic schematic of the silicon photomultiplier, shows the pn-junctions as array of diodes and quenching elements as serial resistors to the individual diodes The process of interaction is shown as photons propagation and. .. (micro-cells) and common output The result was fascinated, first time clear single photon spectra was detected on the semiconductor structure at room temperature Results of study such structures was presented on the 9th European semiconductor conference in 1995 (Saveliev, 1995) 250 Advances in Optical and Photonic Devices And the first concept of Silicon Photomultiplier was proposed fine silicon structure... hole pair inside semiconductor The task of getting controlled avalanche breakdown process consist of providing the very high electric field in limited thickness of semiconductor detecting structure to bring the ionization length of electrons and holes less then the depleted thickness of pn-junctions, and getting required amplification gain with possibility of control by quenching maechanism 3 Principle . developing novel photonic devices. In this chapter, fabrication techniques and characteristics of novel QD photonic devices such as a broadband QD light Advances in Optical and Photonic Devices. novel photonic devices, improvement in the Quantum Dot Photonic Devices and Their Material Fabrications 245 existing photonic devices, and enhancement of usable optical frequency resources in. the ultra-broadband light source between 1 and 1.55 μm, and the novel photonic devices using the cavity-QED. In other words, by using the QD structure, ultra-broadband optical gain media can be